Reference.com. Extremely simple to use, very basically laid out.
Refdesk.com. Includes in-depth research links to breaking news, Word of the Day, reference and Daily Pictures. A fun site with a ton of information.
Encyclopedia.com. As stated on their site, Encyclopedia.com provides users with more than 57,000 frequently updated articles from the Columbia Encyclopedia, Sixth Edition.
Encyclopedia Brittanica. One of the world’s oldest encyclopedias online.
Open Directory Reference. The Open Directory’s guide to various reference sites.
WebReference.com. A great resource for webmasters and anyone else who wants to learn how to develop a webpage.
Purdue University Library Quick Reference. A very good site with tons of info; includes resources specific to Purdue University and surrounding areas in Indiana, USA.
Educator’s Reference Desk. Probably the best reference site online for teachers. Includes thousands of informative links, lesson plans, and general reference information.
Physician Desk Reference. Look for detailed medical information here.
iTools.com. Excellent site; serves as a gateway references and research links.
Baseball-Reference.com. Everything you ever wanted to know about the sport of baseball.
LibrarySpot.com. An excellent site that has hundreds of reference and research sources indexed all in one site.
The Internet Public Library. An invaluable resource that will pretty much take care of all your reference needs.
FOLDOC – Free Online Dictionary of Computing: extremely detailed computing dictionary; I don’t think that there’s a computing term out there that’s not in FOLDOC.
Librarians Internet Index: One of my absolute favorite sites on the Web. You could spend hours here lost in the vast variety of information and resources.